The boundary decides
A potato piece swells in plain water. In salty water, it shrinks. Why?
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The rule
Water crosses the cell membrane toward the saltier side. In plain water, water moves into the potato's cells. In salty water, water moves out.
The cell membrane lets some things pass and blocks others. This is called selective permeability. Water crosses the membrane toward the side with more dissolved substance. This movement of water is osmosis.
Selectively permeable means the membrane lets only some things across. A solute is a dissolved substance, such as salt or sugar. A solution is water with a solute dissolved in it. Osmosis is the movement of water across a membrane.
- In plain water, water moves in. The cell swells.
- In a solution as strong as the cell's, there is no net movement.
- In a stronger solution, water moves out. The cell shrinks.
Three cases
| Solution outside | Water moves | The cell |
|---|---|---|
| Weaker than inside (hypotonic) | In | Swells |
| Same as inside (isotonic) | No net movement | No change |
| Stronger than inside (hypertonic) | Out | Shrinks |
In diffusion, particles spread out. They move from where there are more to where there are fewer. Diffusion needs no membrane. Osmosis is diffusion of water across a selectively permeable membrane.
A potato piece weighs 50 g. After an hour in plain water, the piece weighs more. Water moved in. In strong salt water, the piece weighs less. Water moved out.